The average births per female note, the rise in
teenage pregnancy severely impacts this estimate

The average life expectancy

The annual net immigration rate

Three models (as of 1994):

Birth Rate

Expectancy

Immigration

Projections

1.9

75.3

350,000

293M (2030)
285M (2050)

2.2

82.6

880,000

263(1995)
392M(2050)

2.6

87.5

1.4 million

522M(2050)

In addition to this, the US population is undergoing a significant
regional re-distribution

ZPG is only achieved in a model which limits immigration (a very
politically sensitive issue), restricts birthrates to no more than
2 per female (more politics), and makes no attempt to increase life
expectancy.

The fertility rate has gone up. In 1988 the rate was 1.8 per female,
now its 2.0
this increase is entirely due to teenage pregnancy.

Equilibrium is difficult to achieve because exponential growth produces
demographics such that they are substantially more people entering
their reproductive years than those which are dying of old age.

What about ZPG for the World?

Worst case scenario assumes 2.5 children per couple 28 billion
people by 2150

The above map shows the that the "industrialized" world has stabilized
somewhat to population growth rates less than 1% a year. But
1% is still a doubling time of 70 years and the calculated growth
rates are based, in this study, on only 5 years worth of data.

What does fertility rate depend on? Regression (to be discussed
later) suggests that literacy is a key factor.

Education may be the single biggest contributor towards ZPG.

When did we know there would be a problem?

Historical Estimates of World Population (accurate to 10--20%)

1000 AD 0.25 Billion

1100 AD 0.30

1200 AD 0.36

1250 AD 0.40

1300 AD 0.36 (three cheers for war and disease!)

1350 AD 0.44

1400 AD 0.35

1500 AD 0.43

1600 AD 0.55

1650 AD 0.47

So during this period of 650 years the world population was
stable and fluctuated around a mean value of about 400 million.

Is this the "natural" carrying capacity of the planet?

1700 AD 0.60

1750 AD 0.63

1800 AD 0.82

1850 AD 1.13

Doubling time is approximately 150 years. Some scientists
at the time began to express concern (e.g.
Malthus )

1900 AD 1.55

1910 AD 1.75

1920 AD 1.86

1930 AD 2.07

1940 AD 2.30

1950 AD 2.55

2000 AD 6.50

Since 1900 the population of the world has grown by a factor
of 4 (2 doubling times).

Summary of Population Data:

Clearly in the period 1200-1600 the population of the world
was stabilized. Of course, the quality of life sucked big time.

Between 1900 and 1950 the world population rate grew at about
1% a year as shown below:

But projections based on that growth rate determined for 50 years of
data would have been wrong.

Between 1950 and 1980 world population rate grew at a larger
rate that 1% per year as shown below

In fact, the rate was closer to 1.8% per year which is almost a
factor of 2 shorter doubling time!

Since 1980 there has been a small reduction in population growth
down to about 1.65% per year.

Based on this data is fairly safe to assume, that if conditions do
not change then the doubling time of the worlds population is
40--50 years.

The maximum rate ever observed, about 2.06%, occurred during the
decade of the 60's. This was noticed and UN policy begin to
dealt with the issue. Obviously, however, we are still growing
exponentially at a significant rate.